There was no much need of this feature. In rare cases when we needed to build a bool in Py_BuildValue (I have found only 2 cases in the stdlib, and one of them is added by me) we use the following idiom:

   Py_BuildValue("...O...", ..., expr ? Py_True : Py_False, ...)

You can have a temptation to write it as

   Py_BuildValue("...p...", ..., expr, ...)

but there is a catch -- the arguments should be a C int. If it is not, you can break a stack. Explicit cast to int ("(int)expr") is not always correct, so you will need to write "expr ? 1 : 0" which is not much better than "expr ? Py_True : Py_False".